1. Field of the Invention
The present invention relates to the field of audio electronics, and in particular to interconnect cables and loudspeaker cables.
2. Description of the Related Art
Twisted-pairs of conductors are used for transmission of audio signals because they have inherent noise rejection properties. When a twisted-pair is used for balanced signaling, the two conductors must be identical in length to insure that the signals carried on both conductors arrive in phase at the destination. Equal conductor lengths can improve the signal quality in unbalanced signaling situations as well. This is due to the voltage dividing effect of the forward and return conductors of the twisted-pair. If the voltage drop across the forward path and the return path are not identical across all audio frequencies, then this can cause small, but audible voltage shifts at the cable destination. Unfortunately, achieving equal length of the conductors is difficult and costly with current manufacturing methods. Twisted-pairs are typically fabricated by twisting machines which rotate a frame containing two spools of insulated wire. As the twisted-pair pays out of the twisting machine, the two spools pay out wire under tension in order to equalize their lengths. The spools must have identical tension to maintain uniform twisting and result in equal length of the two conductors. This is process is very difficult to control and systems that control this effectively add significant cost to the twisted-pairs. As a result, the lengths of the conductors in inexpensive twisted-pairs are typically unequal, however one conductor is consistently longer than the other in a given manufacturing run.
Ideally, interconnects for high-fidelity audio transmission should utilize uninsulated conductors. The earliest high-bandwidth cable designs used spacers which supported uninsulated twisted conductors, particularly those used for video and TV transmission. With the advent of low-dielectric constant insulation materials, most cable manufacturers abandoned uninsulated wire designs in favor of these new materials. In some high-bandwidth designs, air is captured within the dielectric material as pockets, tubes or bubbles. Even the best modern insulation materials exhibit undesirable characteristics over the wide bandwidth of audio frequencies. These undesirable characteristics include: high dielectric constant, dielectric absorption and dielectric loss. These characteristics cause the transfer function of the conductors to vary depending upon amplitude and frequency, thereby degrading the signal quality. High dielectric constants also cause high-frequency roll-off by increasing capacitance between the conductors. Air-filled and low dielectric constant materials approach the ideal of air dielectric, but still suffer from frequency dependent effects and generate undesirable noise. Twisted-pair designs could utilize one bare and one insulated wire as an improvement over the typical insulated twisted-pair. However, even in this configuration one conductor experiences the frequency/amplitude-dependent effects of the insulation and the other does not. This creates a varying impedance that causes the signal voltage to vary with frequency at the cable destination, introducing noise into the signal. It is even more difficult to maintain equal conductor lengths when this insulated/uninsulated twisted-pair is manufactured.
Unlike line-level signal cables, cables for the transmission of audio signals between power amplifiers and loudspeakers must transfer high power over a wide range of frequencies without altering the original signal, often into a very reactive loudspeaker load, the impedance varying as a function of frequency. Loudspeakers are generally less than ideal loads for amplifiers to drive due to their inherent inductance and the complex impedance of their crossovers. Amplifiers are often sensitive to the load that the cable/loudspeaker combination presents to them as well and can oscillate or otherwise become unstable if the capacitance or inductance of the cable/loudspeaker combination is too high. In general, obtaining the lowest possible inductance and capacitance is the goal in designing a superior high-fidelity speaker cable. To achieve low inductance, many loudspeaker cable designs simply utilize large conductors, often 10-12 gauge stranded or solid. As the conductors are made larger, their self-inductance decreases. However, this technique is not optimum for audio because these cables experience high-frequency distortion due to skin-effect. When the current density across the cross-section of the conductor varies as a function of frequency, this is skin-effect. The high-frequencies tend to run on the outer skin of the conductor, while the low-frequencies tend run in the center of the conductor. Skin-effect causes the impedance of the conductors and signal velocity to change as a function of frequency and current magnitude.
Most cable designs that minimize skin-effect do so by utilizing "Litz-wire", such as that taught in the inventions of Magnan (U.S. Pat. No. 4,767,890), Low (U.S. Pat. No. 4,997,992) and Brisson (U.S. Pat. No. 4,538,023). In these constructions each conductor typically consists of a large group of individually insulated small-gauge wires, each group having it's ends electrically combined by stripping the insulation. These Litz-wire designs minimize skin-effects by using sufficiently small gauge conductors while the combination of parallel wires reduces the cable inductance. However, even the Litz-wire does not achieve the minimum possible inductance. The inductance of the Litz-wire is limited to the self-inductance of the wire. In some constructions, the fields created by the Litz-wires can couple with other adjacent wires in the bundle increasing or decreasing the impedance of the cable dynamically with the current transients depending upon the magnitude of the currents and associated fields. The impedance can increase or decrease depending upon whether the adjacent wires are coupled by common-mode or differential-mode currents respectively.
Multiple twisted-pair designs such as that taught by Dunlavy (U.S. Pat. No. 5,510,578) minimize skin-effects by utilizing multiple small-gauge conductors that are ganged together like the Litz-wire designs. Twisting the wire pairs tightly together further reduces the inductance of these designs over the non-twisted Litz-wire designs because it creates mutual inductance between the conductors of each pair. Twisted-pairs also have the added advantage of common-mode noise rejection and cancellation of fields at far-field locations from each pair. However, the Dunlavy twisted-pair design locates the pairs in near-field positions with respect to each other with dielectric materials acting as fillers/spacers. In these near-field positions, the magnetic fields from each twisted pair cannot cancel due to flux-lines being broken by close proximity of conductors from other pairs. Capacitive and magnetic coupling can also occur between pairs if they are too close or if they have high dielectric-constant fillers between them. This coupling can cause the cable impedance to fluctuate with high current and voltage transients. These impedance fluctuations can cause phase-shifts in the signals being transferred over the cables which can result in audible distortion.